Semiconductor emitting devices, such as light emitting diodes (LEDs) and laser diodes (LDs), include, but are not limited to, solid state emitting devices composed of group III-V semiconductors. A subset of group III-V semiconductors includes group III-Nitride alloys, which can include binary, ternary and quaternary alloys of indium (In), aluminum (Al), gallium (Ga), and nitrogen (N). Illustrative group III-Nitride based LEDs and LDs can be of the form InyAlxGa1-x-yN, where x and y indicate the molar fraction of a given element, 0≤x, y≤1, and 0≤x+y≤1. Other illustrative group III-Nitride based LEDs and LDs are based on boron (B) nitride (BN) and can be of the form GazInyAlxB1-x-y-zN, where 0≤x, y, z≤1, and 0≤x+y+z≤1.
An LED is typically composed of layers. Each layer has a particular combination of molar fractions for the various elements (e.g., given values of x, y, and/or z). An interface between two layers is defined as a semiconductor heterojunction. At an interface, the combination of molar fractions is assumed to change by a discrete amount. A layer in which the combination of molar fractions changes continuously is said to be graded.
Changes in molar fractions of semiconductor alloys allows for band gap control and are used to form barrier and quantum well (QW) layers. A quantum well comprises a semiconducting layer located between two other semiconducting layers, each of which has a larger band gap than the band gap of the quantum well. A difference between a conduction band energy level of a quantum well and a conduction band energy level of the neighboring semiconductor layers is referred to as a depth of a quantum well. In general, the depth of a quantum well can differ for each side of the quantum well. A barrier comprises a semiconductor layer located between two other semiconductor layers, each of which has a smaller band gap than the band gap of the barrier. A difference between a conduction band energy level of a barrier and a conduction band energy level of a neighboring semiconductor layer is referred to as barrier height. In general, the barrier height of a barrier can differ for each side of the barrier.
A stack of semiconductor layers can include several n-type doped layers and one or more p-type doped layers. An active region of an LED is formed in proximity of a p-n junction where electron and hole carriers recombine and emit light. The active region typically includes quantum wells and barriers for carrier localization and improved radiative recombination. Inside a quantum well, electrons and holes are described quantum mechanically in terms of wave functions. Each wave function is associated with a local energy level inside a given quantum well. An overlap of electron and hole wave functions leads to radiative recombination and light production.
An active region contains multiple quantum wells and barriers. At a quantum well/barrier heterojunction, the lattice mismatch of the two semiconductor layers causes stresses and strains of the crystal layers and leads to the possible formation of cracks, threading dislocations, and other defects.
To decrease a buildup in stresses and strains, multilayered semiconductor superlattices (SLs) have been proposed with layers of alternating compression and tensile stresses. The superlattice structures are used during epitaxial growth of semiconductor layers to minimize the presence of threading dislocations and enable growing thick dislocation free semiconductor layers. Nevertheless, this approach has not been used to minimize stresses in the active light emitting layer.
An alternative approach to minimize lattice mismatch stresses and strains includes growing alternative tensile and compressive layers by varying growing modes for the semiconductor layers. Variation in the V-III ratio and temperature results in growth of compressive and tensile layers. Migration enhanced metalorganic chemical vapor deposition (MOCVD) can be employed (with NH3 pulse-flow) to grow high-quality semiconductor layers to reduce threading dislocations.
FIGS. 1A and 1B illustrate an approach for fabricating AlN multilayer buffers on a sapphire substrate according to the prior art. FIG. 1A shows the gas flow sequence used for ammonia (NH3) pulse-flow growth, while FIG. 1B shows a schematic layer structure of the AlN buffer. In a first step, an AlN nucleation layer and an initial AlN layer are deposited by NH3 pulse-flow growth. A low threading dislocation density was achieved by a coalescence process of the AlN nucleation layer. For example, as observed from a cross-sectional transmission electron microscope (TEM) image, edge-type and screw-type dislocation densities of an AlGaN layer on an AlN buffer layer were reported as 3.2×109 and 3.5×108 cm−2, respectively.